The SEDIGISM survey: The influence of spiral arms on the molecular gas distribution of the inner Milky Way

TL;DR

This study uses the SEDIGISM survey to analyze how spiral arms influence the distribution and properties of molecular gas in the inner Milky Way, revealing similarities with other spiral galaxies and suggesting a flocculent spiral classification.

Contribution

It provides a comprehensive analysis of molecular gas distribution in the Milky Way's spiral arms using large-scale survey data, highlighting key differences and similarities with other galaxies.

Findings

Molecular gas emission in spiral arms is similar to inter-arm regions.

Mass per unit length in spiral arms is about 10^5-10^6 M_sun/kpc.

The Milky Way is classified as a flocculent spiral galaxy.

Abstract

The morphology of the Milky Way is still a matter of debate. In order to shed light on uncertainties surrounding the structure of the Galaxy, in this paper, we study the imprint of spiral arms on the distribution and properties of its molecular gas. To do so, we take full advantage of the SEDIGISM survey that observed a large area of the inner Galaxy in the $^{13}$CO(2-1) line at an angular resolution of 28". We analyse the influences of the spiral arms by considering the features of the molecular gas emission as a whole across the longitude-velocity map built from the full survey. Additionally, we examine the properties of the molecular clouds in the spiral arms compared to the properties of their counterparts in the inter-arm regions. Through flux and luminosity probability distribution functions, we find that the molecular gas emission associated with the spiral arms does not differ significantly from the emission between the arms. On average, spiral arms show masses per unit length of $\sim10^5-10^6$ M$_{\odot} $kpc$^{-1}$. This is similar to values inferred from data sets in which emission distributions were segmented into molecular clouds. By examining the cloud distribution across the Galactic plane, we infer that the molecular mass in the spiral arms is a factor of 1.5 higher than that of the inter-arm medium, similar to what is found for other spiral galaxies in the local Universe. We observe that only the distributions of cloud mass surface densities and aspect ratio in the spiral arms show significant differences compared to those of the inter-arm medium; other observed differences appear instead to be driven by a distance bias. By comparing our results with simulations and observations of nearby galaxies, we conclude that the measured quantities would classify the Milky Way as a flocculent spiral galaxy, rather than as a grand-design one.